diff --git a/src/auto-reply/reply/history.ts b/src/auto-reply/reply/history.ts
index bc59b4f2e..8e1478f76 100644
--- a/src/auto-reply/reply/history.ts
+++ b/src/auto-reply/reply/history.ts
@@ -3,6 +3,26 @@ import { CURRENT_MESSAGE_MARKER } from "./mentions.js";
 export const HISTORY_CONTEXT_MARKER = "[Chat messages since your last reply - for context]";
 export const DEFAULT_GROUP_HISTORY_LIMIT = 50;
 
+/** Maximum number of group history keys to retain (LRU eviction when exceeded). */
+export const MAX_HISTORY_KEYS = 1000;
+
+/**
+ * Evict oldest keys from a history map when it exceeds MAX_HISTORY_KEYS.
+ * Uses Map's insertion order for LRU-like behavior.
+ */
+export function evictOldHistoryKeys<T>(
+  historyMap: Map<string, T[]>,
+  maxKeys: number = MAX_HISTORY_KEYS,
+): void {
+  if (historyMap.size <= maxKeys) return;
+  const keysToDelete = historyMap.size - maxKeys;
+  const iterator = historyMap.keys();
+  for (let i = 0; i < keysToDelete; i++) {
+    const key = iterator.next().value;
+    if (key !== undefined) historyMap.delete(key);
+  }
+}
+
 export type HistoryEntry = {
   sender: string;
   body: string;
@@ -35,6 +55,8 @@ export function appendHistoryEntry<T extends HistoryEntry>(params: {
   history.push(entry);
   while (history.length > params.limit) history.shift();
   historyMap.set(historyKey, history);
+  // Evict oldest keys if map exceeds max size to prevent unbounded memory growth
+  evictOldHistoryKeys(historyMap);
   return history;
 }